1. Field of the Invention
This invention relates to an electron source comprising a large number of electron-emitting devices arranged in a matrix array, an image-forming apparatus comprising such an electron source and a method of driving such an image-forming apparatus.
2. Related Background Art
In recent years, there have been a number of studies on cold cathode type electron-emitting devices, trying to use them for image-forming apparatuses. A surface conduction electron-emitting device is a cold cathode type electron-emitting device. A surface conduction electron-emitting device is realized by utilizing the phenomenon that electrons are emitted out of a small thin film formed on a substrate when an electric current is forced to flow in parallel with the film surface.
A surface conduction electron-emitting device typically comprises an electrically insulating substrate, a pair of device electrodes arranged on the substrate and an electroconductive thin film containing an electron emitting region and arranged between the device electrodes to electrically connect them. The electron emitting region is produced by subjecting the electroconductive thin film, which is typically made of a metal oxide, to a current conduction treatment referred to as energization forming. In an energization forming process, a constant DC voltage or a slowly rising DC voltage that rises typically at a rate of 1V/min. is applied to given opposite ends of the electroconductive thin film to partly destroy, deform or transform the film and produce an electron-emitting region which is electrically highly resistive. When a voltage is applied to an electroconductive thin film where such an electron emitting region is formed in order to make an electric current flow therethrough, the electron emitting region starts emitting electrons.
A surface conduction electron-emitting device having a configuration as described above is advantageous in that it is structurally simple and can be manufactured easily so that a large number of such devices can be arranged over a large area in a simple manner at low cost. Studies have been made to exploit this advantage, and known applications of such devices include image-forming apparatuses including display apparatuses.
The performance of a surface conduction electron-emitting device will be described below by referring to FIG. 19 of the accompanying drawings.
The electric current (If) that flows through a surface conduction electron-emitting device when a voltage (Vf) is applied thereto cannot be uniquely defined. A surface conduction electron-emitting device may operate typically in either of two different ways. Firstly, the electric current flowing through the device (If) may increase in the initial stages as the applied voltage (Vf) is raised from 0[V] but falls thereafter before it gets to a plateau that may be slightly inclined upward. Alternatively, the electric current flowing through the device (If) may monotonically increase as the applied voltage (Vf) is raised from 0[V].
For the sake of convenience, hereinafter, the first characteristic of performance will be referred to as the static characteristic, whereas the second one will be referred to as the dynamic characteristic.
In FIG. 19, the broken line indicates the static characteristic that appears when a voltage sweep speed of less than about 1V/min. is used. More specifically, in the first voltage region of Vf=0 to V1 (I region), the electric current flowing through the device (If) monotonically increases with the increase of the voltage (Vf). In the succeeding voltage region of Vf=V1 to V2 (II region), the electric current flowing through the device (If) decreases with the increase of the voltage (Vf). This characteristic is referred to as a voltage-controlled-negative-resistance characteristic (hereinafter referred to as a xe2x80x9cVCNR characteristicxe2x80x9d hereinafter). In the third voltage region of Vf=V2 to Vd (III region), the electric current flowing through the device (If) practically does not change relative to the increase of the voltage (Vf). Note that V1 represents the voltage when the electric current flowing through the device (If) is maximized and V2 represents the voltage corresponding to the Vf axis intercept of the tangent line to the If curve at the maximum gradient point in the If decreasing resion (II region). Meanwhile, the emission current (Ie) of the device increases as the voltage (Vf) is raised with regard to a threshold voltage Ve.
In FIG. 19, the solid line indicates the dynamic characteristic of the device when the voltage sweep speed is greater than about 10V/sec. More specifically, if the maximum voltage is swept with Vd (If (Vd) line in FIG. 19), the electric current flowing through the device (If) gradually increases and its line comes to agree with the If line for the static characteristic at Vd. If, on the other hand, the maximum voltage is swept with V2 (If (V2) line in FIG. 19), the line of the electric current flowing through the device (If) also gradually increases and its line comes to agree with the If line for the static characteristic at V2. If the maximum voltage is swept with a voltage of the I region, electric current flowing through the device (If) changes substantially along the If line.
While the above described static and dynamic characteristics for the I-V relationship can be varied by changing the material, the profile and/or the other factors of the device or depending on the vacuum atmosphere, a surface conduction electron-emitting device that operates in a desired way typically shows the above three regions, or the regions I through III of performance.
Different electron sources comprising a large number of surface conduction electron-emitting devices arranged in the form of X-Y matrix have been proposed in order to exploit the above described characteristics for flat panel CRTs and other displays.
A matrix type electron source is realized by arranging Mxc3x97N surface conduction electron-emitting devices and electrically connecting them by wires XE1 through XEN and YE1 through YEM as illustrated in FIG. 20 of the accompanying drawings. When such an electron source is used for an image-forming apparatus, e.g. a flat panel CRT, the pixels on the screen and the surface conduction electron-emitting devices are arranged on a one-to-one correspondence basis and the latter are driven to operate according to a given pattern.
Two drive modes are known to date; point-by-point sequential scanning for exciting the screen on a pixel by pixel basis and line-by-line sequential scanning for exciting the screen on a pixel line by pixel line basis. (Each line has M pixels in the arrangement of FIG. 20.) The line-by-line sequential scanning system is normally used as it is advantageous particularly from the viewpoint of the speed of driving each surface conduction electron-emitting device and the momentary current generated by the emitted electron beam because a longer operating time is allocated to each pixel.
Meanwhile, these known scanning systems are accompanied by a problem of high power consumption rate because a large electric current is made to flow to those surface conduction electron-emitting devices that are not currently emitting electron beams and hence staying idle when a large number of surface conduction electron-emitting device are driven either by line-by-line sequential scanning or by point-by-point sequential scanning.
This problem will be discussed below in greater detail by referring to FIGS. 21 through 23 of the accompanying drawings.
FIG. 21 is a schematic plan view of an electron source that comprises only 6xc3x976 surface conduction electron-emitting devices arranged in a simple matrix arrangement for the sake of simplicity. The surface conduction electron-emitting devices are denoted by D(1,1), D(1,2), . . . , D(6,6), using the popular (x,y) coordinate system. If such an electron source is used for a flat panel CRT and each surface conduction electron-emitting device is required to emit an electron beam with a current intensity of 1xc3x9710xe2x88x926A in order to produce a brightness necessary for image display operation, 14V is applied to each of the surface conduction electron-emitting devices that corresponds to a pixel that is emitting light, whereas Vth=10V or less is applied to each of the surface conduction electron-emitting devices that corresponds to a pixel that is not emitting light because of the performance of the surface conduction electron-emitting device shown in FIG. 19.
In order to produce an image on a line-by-line sequential scanning basis, the six device rows running in parallel with the x-axis are sequentially scanned by applying 0V to a row selected out of the six rows of XE1 through XE6 and 7V to the remaining rows that are not selected.
Now, in order to cause any of the surface conduction electron-emitting devices of the selected device row to emit an electron beam with a current intensity of 1 xcexcA, 14V is applied to the wire for feeding the surface conduction electron-emitting device out of the wires YE1 through YE6 and 7V is applied to the remaining wires.
For example, for displaying an image illustrated in FIG. 22, 0V is applied to XE1 and 7V is applied to XE2 through XE6 while 7V is applied to YE1, YE5 and YE6 and 14V is applied to YE2 through YE4 in order to drive the first row. Similarly, 0V is applied to XE2 and 7V is applied to XE1 and XE3 through XE6, while 7V is applied to YE1 and YE3 through YE6 and 14V is applied to YE2 in order to drive the second row. Then, the third through sixth rows are sequentially scanned to produce the image. This operation is summarized in Table 1 below.
With the above drive technique, the surface conduction electron-emitting devices of the unselected rows (unselected devices) may be subjected to a voltage difference of 7V to consequently raise the power consumption rate. Assume that an image of FIG. 22 is being currently displayed and the third device row is being driven, 14V is applied to the opposite terminals of each of the devices at D(2,3), D(3,3) and D(4,3), which by turn emit an electron beam, whereas 14Vxe2x88x927V=7V is applied to the opposite terminals of each of the devices connected to wires YE2, YE3 or YE4 except those on the third row as shown in FIG. 23. As a result, an electric current of 2.5 mA flows through each of the 15 devices of the unselected row at the cost of large power consumption. Thus, it is clear from this example that, when 14V is applied to a surface conduction electron-emitting device, 7V is inevitably applied to each of the surface conduction electron-emitting devices that are commonly wired with that device. While the above electron source comprises only 6xc3x976 surface conduction electron-emitting devices arranged in the form of a matrix for the sake of simplicity, the rate of inutile power consumption will rise enormously in an image-forming apparatus comprising as many as 1,000xc3x971,000 surface conduction electron-emitting devices. Since the power source, the drive circuit and the wires of such an image-forming apparatus have to be selected by taking this large inutile power consumption rate into consideration, the overall cost of such an apparatus can become prohibitive.
In view of the above identified problem, it is therefore an object of the present invention to provide an electron source that can significantly reduce the inutile power consumption of unselected surface conduction electron-emitting devices and, at the same time, effectively avoid unnecessary electron emission that can adversely affect the image forming operation of the electron source. Another object of the invention is provide an image-forming apparatus comprising such an electron source as well as a method of driving such an image-forming apparatus.
According to the invention, the above objects are achieved by providing an electron source comprising a plurality of electron-emitting devices having a pair of electrodes and an electroconductive thin film disposed between the electrodes and containing an electron emitting region and a drive means for driving said plurality of electron-emitting devices, in which:
said drive means applies a voltage above a threshold level to the electrodes of selected ones of said plurality of electron-emitting devices according to an image signal to cause the selected electron-emitting devices to emit electrons and also a voltage pulse for moving said plurality of electron-emitting devices into a high resistance state, said voltage pulse having a polarity reverse to that of the voltage for causing electron emission and a voltage rising (to zero volt) rate (or a falling rate if the absolute value of the voltage is concerned) of greater than 10V/sec.
According to another aspect of the invention, there is provided an image-forming apparatus comprising a plurality of electron-emitting devices having a pair of electrodes and an electroconductive thin film disposed between the electrodes and containing an electron emitting region, a drive means for driving said plurality of electron-emitting devices and an image-forming member, in which:
said drive means applies a voltage above a threshold level to the electrodes of selected ones of said plurality of electron-emitting devices according to an image signal to cause the selected electron-emitting devices to emit electrons and also a voltage pulse for bringing said plurality of electron-emitting devices into a high resistance state, said voltage pulse having a polarity reverse to that of the voltage for causing electron emission and a voltage rising (to zero volt) rate of greater than 10V/sec.